Add response.content_type checks for JSON to api/v1 request specs (#31981)

spec/requests/api/v1/polls_spec.rb

@@ -23,6 +23,8 @@ RSpec.describe 'Polls' do
         subject
 
         expect(response).to have_http_status(200)
+        expect(response.content_type)
+          .to start_with('application/json')
         expect(response.parsed_body).to match(
           a_hash_including(
             id: poll.id.to_s,
@@ -41,6 +43,8 @@ RSpec.describe 'Polls' do
         subject
 
         expect(response).to have_http_status(404)
+        expect(response.content_type)
+          .to start_with('application/json')
       end
     end
   end